1. Field of the Invention
This invention relates to a method and apparatus for measuring the crosstalk of an optical disc to evaluate the quality of the disc.
2. Description of Related Art
Among the items for evaluating the quality of optical discs, such as compact discs (trade mark) or mini-discs (trade mark) is a crosstalk which is used to evaluate the degree of signal deterioration by signal leakage from a neighboring track during reproduction of a specified track.
The crosstalk is defined as B/A, where A is the signal quantity of a specified track and B is the signal leakage from the neighboring track.
Among the methods for measuring the crosstalk of an optical disc, there are hitherto known a reverse tracking method and an off-tracking method.
The reverse-tracking method performs routine signal reproduction in a signal area under measurement to find the signal quantity A, and then reproduces signals between two signal strings in the same area under measurement to measure the stray signal quantity B to find the crosstalk B/A.
With such reverse tracking method, the playback signals are fairly stable and give relatively high repetition accuracy.
However, if the reverse tracking method is to be used, there is required a structure for reproducing signals between two signal strings. This structure is not provided in the usual player. With such reverse tracking method, only the stray signal quantity from a single side of the signal string can be measured, with the result that the measured results differ from the behavior of a real player.
On the other hand, the off-track method utilizes the phenomenon that changes in signal quantities from the signal string generated on disengaging a servo circuit used for following a track (the space lying directly above the track) and from the space between the signal strings (between two tracks) appear alternately. Specifically, the off-track method finds the signal quantity A from the signal string lying directly above the signal string and the stray signal quantity B from the signal quantity from the space between signal strings (space between two neighboring tracks) to find the cross-talk B/A.
With the off-track method, the measurement state can be regenerated by the function proper to the usual player, whilst the signal leakage from both sides of the signal strings can be evaluated which is similar to that encountered during normal reproduction.
However, the off-track method has an inconvenience that, since the difference between the signal quantity from the signal string lying directly above a given track and the signal quantity from the space lying between two neighboring signal tracks, this signal difference signal being called an HF traverse signal, is attributable to the eccentricity occurring at the time of manufacture of the optical disc or the eccentricity when mounting the optical disc on the optical player, which is the eccentricity ascribable to chucking, crosstalk measurement tends to be unstable.
Another inconvenience of the off-track method is that, since the trajectory of the light beam of the optical pickup on servo disengagement is at a predetermined distance from the center of rotation, the crosstalk can be measured only at limited portions of the optical disc, and that, should the optical pickup be set on the turntable in a different manner, the path followed by the light beam of the optical pickup is varied to change the measured values, rendering the measured values unstable.
Moreover, the off-track method has an inconvenience that, should the amount of eccentricity on mounting the optical disc on the player be small, the chance of the light beam of the optical pickup traversing the signal string (track) of the optical disc decreases, meaning that measurement only at limited portions of the optical disc lying on the light beam trajectory is used to represent the measurement for the entire area of the optical disc. In addition, should there be defects of the optical disc on the light beam trajectory, variations in crosstalk measurement undesirably tend to increase.
It is therefore an object of the present invention to provide a method and apparatus for measuring the crosstalk of an optical pickup in which the crosstalk of the optical disc can be measured with stability.
In one aspect, the present invention provides a method for measuring the crosstalk of an optical disc in which there are concentrically or spirally formed continuous recorded signal strings or signal recording regions and in which there is provided a positioning aperture at the center of the disc substantially concentrically with the recorded signal strings or signal recording regions. A light beam of an optical pickup of an optical disc player is wobbled at a pre-set amplitude along the radius of the optical disc, with the wobbling period asynchronous with respect to the rotational period of the optical disc. The ratio of the signal quantity of traverse signals obtained by the optical pickup and the signal quantity of the stray signals is measured as a crosstalk.
With the measurement method according to the present invention, in which, for measuring the crosstalk of the optical disc, the light beam of the optical pickup is wobbled at a predetermined amplitude in a radial direction of the optical disc, it is possible for the light beam to traverse e.g., 32 to 80 signal strings (tracks) per rotation of the optical disc to provide stable HF traverse signals. Moreover, since the wobbling period is adapted to be asynchronous with the period of rotation of the optical disc, the light beam has its trajectory changed from time to time without following the same trajectory to enable optimum crosstalk measurement over the entire width of the pre-set amplitude along the radius of the optical disc.
In another aspect, the present invention provides an apparatus for measuring the crosstalk of an optical disc in which there are concentrically or spirally formed continuous recorded signal strings or signal recording regions and in which there is provided a positioning aperture at the center of the disc substantially concentrically with the recorded signal strings or signal recording regions. The apparatus includes wobbling means for wobbling a light beam of an optical pickup of an optical disc player at a predetermined amplitude and at a period asynchronous with the period of rotation of the optical disc, in a radial direction of the optical disc, and calculating means for calculating the ratio of the signal quantity of traverse signals obtained by the optical pickup and the signal quantity of the stray signals as a crosstalk.
With the apparatus for measuring the crosstalk of the optical disc according to the present invention, in which the light beam of the optical pickup is wobbled a predetermined amplitude along the radius of the optical disc in measuring the crosstalk of the optical disc, 32 to 80 signal strings (tracks), for example, can be traversed by the light beam for each revolution of the optical disc, thus producing stable HF traverse signals. In addition, since the wobbling period is asynchronous with the period of disc rotation, the light beam of the optical pickup has its trajectory changed as time elapses, when the optical disc has made 10 complete revolutions, as an example, thus enabling optimum crosstalk measurement over the entire range of the predetermined amplitude along the radius of the optical disc.